We have observed that the administration of 1,25 - dihydroxycholecalciferol (1,25 (OH)2 D3) to rats induces a hypoprothrombinemic state, stimulates the hepatic vitamin K-dependent gamma glutamyl carboxylase, and induces accrual of both proteinaceous substrates for the carboxylase and of Echis carinatus detectable thrombin activity within the endoplasmic reticulum. As far as we know, these effects are the first in the liver ascribed to vitamin D. The mechanisms for these effects are unknown. We propose to clarify these mechanisms by the study of plasmas and appropriate hepatic preparations derived from control, warfarin treated, and 1,25 (OH)2D3-treated rats. Assays to be conducted include clotting assays, carboxylase assays (endogenous as well as peptide carboxylation), radioimmunoassays of prothrombin, gamma-carboxyglutamic acid (Gla) determinations including Gla content of various prothrombins, carbohydrate measurements, SDS polyacrylamide gel electrophoresis of vitamin K-dependent 14C-labeled hepatic microsomal proteins. Experiments will also be conducted to determine if the 1,25 (OH)2D3 effects are mediated via increased calcium concentrations. Finally, it will be determined if the prothrombin-producing Reuber H-35 hepatoma cell line is responsive to 1,25 (OH)2D3 or increased intracellular calcium.